1. Field of the Present Disclosure
The present disclosure relates to an image forming apparatus.
2. Description of the Related Art
In general, in an electrographic image forming apparatus, even if dots are increased in accordance with a pixel value in an image, a toner density is not in proportion to the pixel value.
Further, a relationship between the pixel value and the toner density varies due to an individual difference of image forming apparatuses, an environmental condition and the like, and therefore it is required to adjust the density characteristic of each individual apparatus in order to perform printing with a toner density proportional to a pixel value.
In a density adjustment method, an image forming apparatus prints a test chart on a paper sheet, scans the test chart printed on the paper sheet using a scanner or the like, and generates gradation correction data on the basis of the scan result. However, in this method, manual operations for the printing and the scanning of the test chart are bothersome, and therefore, an image forming apparatus often automatically adjusts the gradation correction data at regular intervals. In such a case, the image forming apparatus forms an adjustment toner pattern on an intermediate transfer member, measures a density of the adjustment toner pattern using a density sensor, and adjusts the gradation correction data on the basis of the measurement result.
In case of adjusting the gradation correction data on the basis of the measurement result of the density of the adjustment toner pattern on the intermediate transfer member, the adjustment toner patter is formed on the basis of a corrected pixel value using the gradation correction data before the adjustment thereof; and consequently, when reducing a toner density due to aging or the like, even though the density adjustment is performed, a maximum density of the toner density sometimes does not reach to a reference maximum density (e.g. ID (Image Density)=1.4).
FIGS. 8A to 8D show diagrams that explain adjustment of gradation correction data. FIG. 8A shows a diagram that indicates an example of a relationship between a pixel value and a toner density when generating gradation correction data by using a test chart. FIG. 8B shows a diagram that indicates a goal relationship between a pixel value and a toner density. FIG. 8C shows a diagram that indicates gradation correction data obtained from the relationship between a pixel value and a toner density shown in FIG. 8A and the goal relationship shown in FIG. 8B. FIG. 8D shows a diagram that indicates a relationship between a pixel value and a toner density after the density characteristic changes.
For example, in the density characteristic shown in FIG. 8A, if the gradation correction data is generated so that (a) the toner density gets a reference maximum density at a maximum value Xmax of a pixel value (e.g. 255 for 8 bit data) and (b) a toner density is in proportion to a pixel value, then the gradation correction data is as shown in FIG. 8C.
Afterward, when the density characteristic varies as shown by the solid line in FIG. 8D and the density decreases from the characteristic shown in FIG. 8A (i.e. the dashed line in FIG. 8D), if an adjustment toner pattern is generated using the gradation correction data shown in FIG. 8C, then as shown in FIG. 8D, the density at Ymax corresponding to the maximum value Xmax of a pixel value (i.e. a maximum density after the gradation correction) gets smaller than the reference maximum density.
In order to solve the aforementioned problem, in an image forming apparatus, a correction data generating unit (a) measures a density characteristic, (b) generates temporary gradation correction data so as to convert a maximum density of the density characteristic to a reference maximum density lower than the maximum density and make the density characteristic linear, (c) generates basic gradation correction data so as to make the density characteristic linear with keeping the maximum density of the density characteristic, and (d) generates additional gradation correction data used for additional gradation correction performed after gradation correction based on the basic gradation correction data so that gradation correction based on the basic gradation correction data and the additional gradation correction data gets the same as gradation correction based on the temporary gradation correction data; and an image processing unit performs gradation correction of a printing image on the basis of the basic gradation correction data and the additional gradation correction data. Consequently, the obtained correction data can be used up to the reference maximum density, even if the density characteristic varies due to adjustment of the additional gradation correction data.
In an image forming apparatus capable of printing at plural printing resolutions, the printing is performed at any one of plural different process linear velocities (printing linear velocities of full speed, half speed, ¼ speed and the like) corresponding to the plural printing resolutions.
Different process linear velocities result in different density characteristics (i.e. maximum densities and gradations in an image), and therefore when performing calibration to adjust the density characteristic, it is required to perform calibration plural times for plural process linear velocities; and however long time is required to perform the calibration for plural process linear velocities, and therefore it is not practical.
Therefore, in general, calibration is performed at a reference linear velocity, and an adjustment amount obtained by the calibration is also applied to a density characteristic at another linear velocity.
However, when applying the adjustment amount obtained by the calibration at the reference linear velocity to a density characteristic at another linear velocity, the density characteristic at another linear velocity may not be properly adjusted.
FIG. 9 shows a diagram that indicates an example of density characteristics at plural linear velocities. For example, as shown in FIG. 9, if due to aging, a maximum density at a reference linear velocity (here, full speed) corresponding to 600 dpi exceeds a reference maximum density, but a maximum density at a linear velocity (here, half speed) corresponding to 1200 dpi is less than the reference maximum density, then in the calibration at the reference linear velocity, the density characteristic at the reference linear velocity is adjusted so as to improperly decrease an image density. In this situation, if an adjustment amount obtained by the calibration of the density characteristic at the reference linear velocity corresponding to 600 dpi is applied to the density characteristic at the linear velocity corresponding to 1200 dpi, then despite the maximum density less than the reference maximum density, the density characteristic at this linear velocity is adjusted so as to improperly decrease an image density and consequently the maximum density improperly gets much lower than the reference maximum density.